Template-Free Synthesis of Mesoporous β-MnO₂ Nanoparticles: Structure, Formation Mechanism, and Catalytic Properties

Mesoporous β-MnO₂ nanoparticles were synthesized by a template-free low-temperature crystallization of Mn⁴⁺ precursors (low-crystallinity layer-type Mn⁴⁺ oxide, c-distorted H⁺-birnessite) produced by the reaction of MnO₄^- and Mn²⁺. The Mn starting materials, pH of the reaction solution, and calcination temperatures significantly affect the crystal structure, surface area, porous structure, and morphology of the manganese oxides formed. The pH conditions during the precipitation of Mn⁴⁺ precursors are important for controlling the morphology and porous structure of β-MnO₂. Nonrigid aggregates of platelike particles with slitlike pores (β-MnO-1 and -2) were obtained from the combinations of NaMnO₄/MnSO₄ and NaMnO₄/Mn(NO₃)₂, respectively. On the other hand, spherelike particles with ink-bottle shaped pores (β-MnO-3) were formed in NaMnO₄/Mn(OAc)₂ with pH adjustment (pH 0.8). The specific surface areas for β-MnO-1, -2, and -3 were much higher than those for nonporous β-MnO₂ nanorods synthesized using a typical hydrothermal method (β-MnO-HT). On the other hand, c-distorted H⁺-birnessite precursors with a high interlayer metal cation (Na⁺ and K⁺) content led to the formation of α-MnO₂ with a 2 × 2 tunnel structure. These mesoporous β-MnO₂ materials acted as effective heterogeneous catalysts for the aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) as a bioplastic monomer and for the transformation of aromatic alcohols to the corresponding aldehydes, where the catalytic activities of β-MnO-1, -2, and -3 were approximately 1 order of magnitude higher than that of β-MnO-HT. β-MnO-3 exhibited higher catalytic activity (especially for larger molecules) than the other β-MnO₂ materials, and this is likely attributed to the nanometer-sized spaces.